AI has no clue what it's talking about.
Code:
ldd html_parser.cpython-314-x86_64-linux-gnu.so| grep xml
libxml2.so.16 => /usr/lib/libxml2.so.16 (0x00007fd29e127000)
html5-parser does not use load library.
And I tried, once again, to reproduce this issue in a fresh debian:stable container which has libxml.so.2 installed and it does not reproduce.
/opt/calibre/ebook-convert test.txt .pdf
works as expected.